The present invention relates to fuel systems in automotive vehicles. More specifically, this invention relates to a diagnostics system for detecting leaks in a fuel system for an automobile engine.
On-board diagnostics for detection of fuel system leaks have been required in the United States since Model Year 1996 by both the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB). Leaks equivalent to a 0.040 inch (1 mm) diameter hole or greater anywhere in the fuel system are currently required to be detected by the EPA, while CARB lowered the detection level requirements to 0.020-inch diameter holes for the Model Year 2000.
Two methods of leak detection have generally been used, namely, vacuum decay and pressure decay. Vacuum decay methods typically have a cost advantage over pressure based systems; however, vacuum decay methods have been thought to be deficient with respect to their ability to reliably detect 0.020 inch leaks.
One deficiency in previous vacuum-based evaporative leak diagnostic systems is that high purge rates required to evacuate the fuel tank at idle cannot be achieved. This is due to either insufficient fuel injector or integrator margins to allow the necessary purge rates.
Another deficiency in the prior systems is that the lower purge rates results in either longer idle times required to evacuate the fuel tank or in not being able to draw the required tank volume for certain types of fuel and leak combinations.
A third deficiency in prior systems diagnostics is that idle stability problems occurred when purge solenoid valves are closed for purge duty cycles which are greater than 10% to 40% at idle. The purge duty cycle is a software calculation that determines how long the purge solenoid valve is opened during one pass through the software.
It is therefore an object of the present invention to integrate an enhanced purge function algorithm into an enhanced vacuum decay diagnostic that compensates for these deficiencies by adding unique purge duty cycle rates and limits to allow for the higher amount of purge necessary to draw the required fuel/leak combinations and to allow for the higher purge duty cycle transient rates required to shut off purge when the tank vacuum target is reached to minimize vacuum overshoots.
To accomplish this, three purge modes (TANK, MASS FLOW, and RAMP OFF) are used to support the evaporative diagnostic. TANK mode modifies the purge logic to support the aggressive purging requirements of a Preset Large Leak Test, a Warm Large Leak Test, and an Idle Large Leak Test. MASS FLOW mode modifies the purge logic to hold a constant purge flow mass rate that is necessary during the Vent Blockage Test. RAMP OFF mode increases the ramp down rate of the purge duty cycle to aggressively shut off the purge value at the start of the Small and Very Small System Leak Tests and clamps purge off during the Purge Valve Leak Test.
In one aspect of the present invention, the evaporative diagnostic determines whether small or large leaks are present in the fuel system and whether the vent solenoid valve is blocked or partially blocked by performing tests using the three purge modes (RAMP OFF, TANK, and MASS FLOW) when certain engine operating conditions are present.
In a further aspect of the invention, the RAMP OFF mode is used in conjunction with the Small and Very Small Leak Tests to determine whether leaks as small as 0.02 inches in diameter are present in the fuel system. The test comprises the steps of determining whether a set of engine operating conditions is present; drawing a predetermined vacuum in the fuel system; sealing the fuel system; allowing the vacuum to decay for a predetermined amount of time; and indicating when said the pressure decay exceeds the predetermined vacuum decay threshold.
In a further aspect of the invention, the TANK mode is used in conjunction with the Warm, Preset and Idle Large Leak Tests to determine whether large leaks of greater than 0.04 inches in diameter are present in the fuel system. The test comprises the steps of determining whether a set of engine operating conditions is present; closing a vent solenoid valve; drawing a vacuum across the fuel system at a predetermined rate for a predetermined time; determining whether a vacuum pressure rise exceeds a predetermined vacuum rise threshold; or indicating when the vacuum pressure rise is less than the predetermined vacuum rise threshold within a predetermined time.
In a further aspect of the invention, the MASS FLOW mode is used to determine whether there is a blockage or partial blockage in the vent solenoid valve of the fuel system. The test comprises the steps of determining whether a set of engine operating conditions is present; opening the vent solenoid valve and a purge solenoid valve of the fuel system; purging the fuel system at a predetermined constant rate until a sufficient mass is purged; determining whether a vacuum pressure rise exceeds a predetermined vacuum rise threshold; or indicating when the vacuum pressure rise exceeds the predetermined vacuum rise threshold within a predetermined time.
Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings.